fix(telephony): resolve transfer context via call-sid index instead of KEYS scan (#387)

Transfer-context lookup by original_call_sid ran
`redis.keys("transfer:context:*")` and iterated every match — an O(N)
blocking scan on call-control hot paths, duplicated across the ARI
manager and the Twilio/Telnyx conference strategies.

Maintain a `transfer:by_call_sid:{original_call_sid}` -> transfer_id
secondary index, written and cleared alongside the context in
store/remove, and resolve lookups with a direct GET. Route the
Twilio/Telnyx strategies through the manager so the lookup lives in one
place (also dropping per-call ad-hoc Redis connections).

Closes #328

Co-authored-by: shiminshen <16914659+shiminshen@users.noreply.github.com>

api/services/telephony/transfer_event_protocol.py

@@ -99,3 +99,12 @@ class TransferRedisChannels:
     def transfer_context_key(transfer_id: str) -> str:
         """Redis key for transfer context storage."""
         return f"transfer:context:{transfer_id}"
+
+    @staticmethod
+    def transfer_context_by_call_sid_key(original_call_sid: str) -> str:
+        """Redis key for the original_call_sid -> transfer_id secondary index.
+
+        Lets a caller's transfer context be resolved with a direct lookup
+        instead of an O(N) ``KEYS transfer:context:*`` keyspace scan.
+        """
+        return f"transfer:by_call_sid:{original_call_sid}"